1. Field of the Invention:
A process for recovering ammonia from mixtures containing ammonia contaminated with CO.sub.2.
2. Discussion of the Background
In a number of chemical processes, ammonia is produced as a byproduct or co-product, and in the form in which it must be withdrawn from the process it is contaminated with organic products and greater or lesser amounts of CO.sub.2. Examples of such processes are the manufacture of O-carbamates from urea and alcohol, or the manufacture of N-substituted urethanes from urea, amine, and alcohol. Even in reactions where ammonia is a reaction component, it is often necessary to withdraw offgases in order to prevent a build-up of by-products and/or inert components. Carbon dioxide is also frequently found in these ammonia-containing offgases, which complicates or renders impossible the processing and recovery of valuable substances because of formation of ammonium carbamate which separates out as a solid onto heat exchanger surfaces or leads to plugging of tubes. Therefore, frequently these gases are not further processed to recover the valuable product but rather are disposed off by burning. There is a clear trend toward use of such waste substances as materials other than fuel, which trend is furthered by the fact that the environmental permits for the processes often contain such use as a condition. Because permit variances for thermal disposal of ammonia-containing gases are very difficult to obtain, there is a demand for suitable processes for recovering ammonia and other valuable substances from CO.sub.2 -containing offgases.
Carbon dioxide and ammonia form ammonium carbamate according to the following equation EQU 2NH.sub.3 +CO.sub.2 .revreaction.NH.sub.2 --CO--O.sup.- NH.sup.+ .sub.4.
The equilibrium of this equation was determined by Bennett, R. N., et al., Trans. Farad. Soc., 49 (1953), 925, and Janjic, D., Helv. Chim. Acta, 47 (1964), 1879. According to these determinations, ammonium carbamate at 1 bar, under its own vapor pressure, is stabile to about 60.degree. C.
Because the dissociation constant is EQU K.sub.p =P.sub.NH.sbsb.3.sup.2 .multidot.P.sub.CO.sbsb.2
where P.sub.NH.sbsb.3 and P.sub.CO.sbsb.2 are the partial pressure of ammonia and carbon dioxide, respectively, for each combination of partial pressures one can determine the decomposition temperature of the ammonium carbamate and adjust the process conditions such that no solid ammonium carbamate is deposited.
Because ammonium carbamate is nearly insoluble in liquid ammonia, ammonia cannot be liquified in the presence of carbon dioxide without deposition of substantial quantities of ammonium carbamate at the surface of the heat exchanger used for the condensation.
Of course, one can operate at temperatures below the decomposition temperature providing a second condenser, operable in parallel with the first, such that when the first condenser becomes so fouled with ammonium carbamate that it can no longer perform the required cooling, the second condenser is switched in. However, this method of operation requires additional investment, and additional costs of monitoring and removing the fouling deposits, and has the further disadvantage that the ammonia thus obtained is not free of organic impurities and thus is not reusable.
Thus, in U.S. Pat. No. 3,013,065 the vapors arising in the manufacture of ethyl carbamate by reaction of urea and ethanol at 6-7bar undergo condensation at, alternately, 90.degree. C. and 20.degree. C. At 20.degree. C. the condenser becomes covered with ammonium carbamate. The NH.sub.3 after flash evaporation then still contains about 2 wt. % ethanol. An additional disadvantage of the method is that the contents of the first condenser after switching over to the parallel (second) condenser include not only solid ammonium carbamate but also the gas phase, which can only be disposed of at substantial cost (in terms of apparatus), e.g. by flash evaporation, subsequent compression, and recycling to the system.
In Austrian Pat. 261,638 and in Chem. Ing. Techn., 42 (1970), 521, a method is described which is specially designed to process the offgases of melamine manufacture. Here CO.sub.2 and NH.sub.3 are recovered in pure form by first washing out the CO.sub.2 at ambient pressure and low temperature, using aqueous ammonia solution, to produce ammonium carbonate, while ammonia (saturated with water) is obtained at the top of the washer. The ammonia is then dried by low temperature rectification. The ammoniacal ammonium carbonate solution obtained at the bottom of the washer is sent to a stripper column, but only part of the ammonia dissolved in the solution can be recovered in the column because of formation of a constant-boiling mixture. In a subsequent pressure distillation, the CO.sub.2 liberated by thermal decomposition of the ammonium carbonate is drawn off as an overhead product; ammonium carbonate which deposits in the upper part of the column is dissolved by irrigating with water. Part of the bottoms product is sent to a second distillation where CO.sub.2 and NH.sub.3 are removed; these are recycled to the first washing stage, and the water is withdrawn.
Disadvantages connected with this process are the separation of the ammonia at ambient pressure and its liquification at low temperature, the large water recycle with its attendant high energy consumption, and the high investment costs (six columns are needed). The process is unsuitable for treating offgases which also contain organic products because approximately 20 wt. % ammonia solution remains after the (incomplete) removal of the CO.sub.2, and the organic components must then be removed from this solution.
The use of amines in aqueous solution to wash CO.sub.2 from gases is a technique well known in the art, e.g. for gas purification in ammonia synthesis "Ullmann's Encyclopedia of Industrial Chemistry", 5th Ed. (1985), Vol. A2, p. 180). A disadvantage, beside the introduction of water, is that the organic materials present in the offgases are dissolved to some extent in the absorption liquid, and can be separated from that liquid only by relatively costly means.
Theoretically, based on the diameters of the CO.sub.2 and NH.sub.3 molecules known from the literature, CO.sub.2 should be selectively adsorbed onto a 3-Angstrom molecular sieve. However in tests it was found that NH.sub.3 is adsorbed as strongly as CO.sub.2, so that the CO.sub.2 cannot be selectively separated and so removed from offgases.
According to Jaenecke, E., Z. Elektrochem, 35 (1929), 716, the ammonia/ammonium carbonate system provides two liquid phases in addition to solid ammonium carbonate, at temperatures above 118.5.degree. C. This observation is of little value for industrial purposes, however, because the pressure required is very high, and because ammonium carbonate is still present in solid form; also, the temperature required is close to the critical temperature of the ammonia.
It is possible to employ a method whereby ammonia is washed out with dilute sulfuric acid, with organic components being separated out from the filtrate. This method does not present major technical problems, but is nonetheless uneconomical, because ammonium sulfate is already unavoidably produced in a large number of industrial processes, so that market supply greatly exceeds demand. Examples of such processes are the Beckmann rearrangement to produce caprolactam and the production of methacrylic acid from acetone cyanohydrin.